I think the power transistors you are using are high voltage, low gain types. So they need a lot of base current to get fair saturation. If you replace them with a low resistance N-channel mosfet, that should run much cooler. They are also much simpler to connect to the 555. The gate can be connected directly to the 555 output.
Go to Digikey and find the data sheets for IRFZ44V, IRF540, IRF3704 for examples that should work. One of these can handle your load with no heat sink.
-- John Popelish
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Are you using the transistors in parallel? What frequency is your 555 running at? Duty Cycle? Schematic?
All that aside, worst case gain for that transistor is only 8 (Ic=2A) which means you will need to supply at least 300mA (each) of drive into the base to turn the transistors on all the way. AFAIK, the 555 can't supply that much current by itself. What you need is a nice MOSFET, preferably one that uses a "logic level" input.
one I am
I need to control a simple heating element which will run off 12Vdc. It does not need to get very hot.
I need to make it variable. At the moment I have a 555 running in astable mode pulsing two MJ13005 transistors which are directly connected to the element. The transistors seem to get too hot. What other methods can I use?
My element usualy are a bunch of fine stranded copper wires and the one I am currently using has a resistance of 5ohms.
I need to run this off 12Vdc.
Couldn't find much on your transistors.
2 things, Use a better transistor like a MOSFET that has a low switched on resistance. 2nd, use the pulse width function of the 555 to adjust the temp. you will only need one transistor instead of 2 drives. A predrive may be needed. You are running about 2.5 amps There are many devices out there that would work I would get one about 5 amps and a med. heat sink. Mr. Eric SpainElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.